Method, systems and sensor for detecting humidity

ABSTRACT

Methods systems and device for detecting humidity in air through use of an ammonia sensor included in the exhaust of an engine, such as a diesel engine are provided. In one example, a method for an engine having an exhaust with an ammonia sensor includes adjusting an operating parameter in response to ambient humidity, the ambient humidity based on a first ammonia sensor reading at a first exhaust air-fuel-ratio and a second ammonia sensor reading at a second exhaust air-fuel-ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 61/230,518, entitled “Method, Systems and Sensor forDetecting Humidity,” filed Jul. 31, 2009, the disclosure of which ishereby incorporated by reference in its entirety and for all purposes.

TECHNICAL FIELD

The present application relates to adjusting various operatingparameters in response to ambient air humidity, the humidity identifiedbased on an ammonia sensor included in the exhaust of an engine, such asa diesel engine.

BACKGROUND AND SUMMARY

Ammonia sensors may be used in exhaust emission control to maintainaccurate control of injected reductant, such as injected urea. However,the inventors herein have recognized that depending on the ambienthumidity, the ammonia reading from the sensor may be erroneous.

However, the inventors herein have also recognized that the errors dueto ambient humidity manifest themselves in a predictable way acrossdifferent air-fuel ratios. Further, the inventor have developed variousapproaches that correlate the sensor readings under selected conditionsto ambient humidity. As such, the sensitivity to ambient humidity may beused to advantage, rather than viewed as purely a detrimental effect.

In one example, a method for an engine having an exhaust with an ammoniasensor includes adjusting an operating parameter in response to ambienthumidity, the ambient humidity based on a first reading of the ammoniasensor at a first exhaust air-fuel-ratio and a second reading of theammonia sensor at a second exhaust air-fuel-ratio.

In another example, a method using an ammonia sensor in an engineexhaust SCR system, includes in a first mode, identifying ambienthumidity by measuring an ammonia concentration via the ammonia sensorduring two different air-fuel-ratios, the measuring in response to anindication of a potential humidity change and may further include in asecond mode, controlling an ammonia or urea injection dosage viafeedback from the ammonia sensor, where during the first mode, ammoniaor urea injection dosage is independent of current ammonia sensorfeedback and based on previous ammonia sensor feedback. In still anotherexample, a method for an engine having an ammonia sensor in an exhaustdownstream of an SCR catalyst, includes during a first mode, adjustingammonia injection in response to the sensor, and during a second mode,adjusting ammonia injection independent of the sensor while indicatingambient humidity based on the sensor.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example diesel engine including a selective catalyticreduction system with an ammonia sensor;

FIG. 2 shows an exploded view of an example ammonia sensor;

FIG. 3 shows an example high level routine for controlling ammonia andhumidity concentration measurements using an ammonia sensor;

FIG. 4 shows an example routine for controlling ammonia dosing to an SCRcatalyst;

FIG. 5 shows an example routine for measuring humidity via an ammoniasensor;

FIG. 6 is a graph illustrating the relationship between error in NH3ammonia reading measured by an ammonia sensor and air-to-fuel ratio forvarious amounts of humidity.

DETAILED DESCRIPTION

FIG. 1 shows an example diesel engine 10 including a selective catalyticreduction (SCR) system 50 with an ammonia sensor 200. Though the examplepresented is a diesel engine, reducing and controlling engine emissions,particularly NOx, is an important consideration in modern internalcombustion engines, both spark-ignited and compression-ignited.Therefore, the systems, devices and methods described herein may beincluded in any exhaust system that has a reductant based SCR system.

FIG. 1 further shows engine 10 including an intake 23, an exhaust 25, afuel system 18, an exhaust gas recirculation (EGR) system 26, and acontrol system 14. The engine 10 has a plurality of cylinders 30. Engine10 may be controlled at least partially by control system 14 includingcontroller 12, as well as by input from a vehicle operator 132 via aninput device 130. In this example, input device 130 includes anaccelerator pedal and a pedal position sensor 134 for generating aproportional pedal position signal.

The engine 10 includes an engine intake system 23 and an engine exhaustsystem 25. The engine intake 23 includes a throttle 62 fluidly coupledto the engine intake manifold 44 via an intake passage 42. In additionalexamples, e.g., diesel engines, throttle 62 may not included in engine10. Intake passage 42 includes mass air flow (MAF) sensor 120 as well asambient air temperature 121 and intake manifold 44 further includesmanifold air pressure sensor 122. In additional examples, the engine 10further includes an air induction system (AIS) (not shown), the AIShaving an air filter and housing, various vapor purging valves such asfrom an engine crankcase or the fuel system 18, etc.

The engine exhaust system 25 includes an exhaust manifold 48 leading toan exhaust passage 35 that routes exhaust gas to the atmosphere. Theengine exhaust system 25 includes sensors, for example, exhaust gassensor 126 which may be an oxygen or lambda sensor, temperature sensor128 and pressure sensor 129. The engine exhaust system 25 also mayinclude one or more emission control devices 70, which may be mounted ina close-coupled position in the exhaust. One or more emission controldevices may include a three-way catalyst, lean NOx trap, dieselparticulate filter, diesel oxidation catalyst, hydrocarbon trap, etc.Further, in the present example, an SCR system 50 and EGR system 26 areincluded in the engine.

Engine 10 further includes turbocharger 180. Turbocharger 180 includes acompressor 182, schematically shown as linked to turbine 184 via turboshaft 186. Additionally, turbocharger 180 may be a supercharger, lackingturbine 184 and may be mechanically linked to a crankshaft. Furtherstill, compressor 182 may be driven, at least partially by an electricmotor (not shown).

SCR system 50 includes a urea tank 52, coupled to urea pump system 54,which is further coupled to urea injector 56. The urea injector deliversurea into the exhaust system 25 upstream of a SCR catalyst 58 whichreceives the urea. The urea pump system 54 may include one or more pumpsfor pressurizing urea delivered to urea injector 56. Further example SCRsystems may include more than one urea injector. After urea is deliveredinto the exhaust 25 it may convert to ammonia. Although the presentexample shows a urea-based system, in additional examples, any reductantmay be used including ammonia and a urea and ammonia blend. SCR catalyst58 may catalyze a reaction of ammonia with NOx to yield diatomicnitrogen and benign by-products such as water.

Ammonia sensor 200 is included downstream of SCR catalyst 58 to senseammonia that travels downstream of SCR catalyst 58, and will bediscussed in more detail below with regard to FIG. 2. Additionally, SCRsystem 50 may include a slip catalyst (not shown) located downstream ofSCR catalyst 58. The slip catalyst may mitigate ammonia slip from SCRcatalyst 58. The slip catalyst may catalyze one or both of a reaction tooxidize ammonia and a reaction to reduce NOx. Further a second ammoniasensor, of a similar type as ammonia sensor 200, may be includeddownstream of such an SCR slip catalyst, if desired.

EGR system 26 includes an EGR pipe 28 and an EGR valve 29. EGR pipe 28directs, at least partially, exhaust gas flow from exhaust passage 35back to intake passage 42. In the present example, EGR pipe 28 is showncoupled to exhaust passage 35 downstream of SCR system 50. However, inadditional examples, EGR pipe 28 may be coupled to exhaust passage 35upstream of SCR system 50, as well as upstream of emission controldevice 70 and also directly to exhaust manifold 48. In still furtherexamples EGR pipe 28 may couple directly to intake manifold 44. EGRvalve 29 may be an on/off valve or variable valve. EGR system 26 mayalso include a device coupled to EGR pipe 28, or placed intermediatelyalong EGR pipe 28 for cooling air within EGR pipe 28, such as anintercooler (not shown).

Fuel system 18 may include a fuel tank 20 coupled to a fuel pump system21. Fuel system 18 and/or throttle 62 may control a ratio of air-to-fuelinducted in to the engine 10. The fuel pump system 21 may include one ormore pumps for pressurizing fuel delivered to the injectors of engine10, such as the example injector 66 shown. While only a single injector66 is shown, additional injectors may be provided for each cylinder orfuel may be injected into the manifold 44. It can be appreciated thatfuel system 18 may be a return-less fuel system, a return fuel system,or various other types of fuel system. The fuel tank 20 may hold aplurality of fuels and fuel blends, including fuel with a range ofalcohol concentrations.

Example engine 10 further includes control system 14. Control system 14is shown receiving information from a plurality of sensors 16 (variousexamples of which are described herein) and sending control signals to aplurality of actuators 81 (various examples of which are describedherein). As one example, sensors 16 may include exhaust gas sensor 126,temperature sensor 128, pressure sensor 129, pedal position sensor 134,MAF sensor 120, MAP sensor 122, ambient air temperature sensor 121, andammonia sensor 200. Other sensors such as pressure, temperature,air-to-fuel ratio, and composition sensors may be coupled to variouslocations in the engine 10. As another example, the actuators mayinclude fuel injector 66, reductant injector 56, EGR valve 29, andthrottle 62. The control system 14 may include a controller 12, thecontroller 12 including computer readable storage medium, such as ROM orRAM memory, with instructions thereon. The controller may receive inputdata from the various sensors, process the input data, and trigger theactuators in response to the processed input data based on instructionor code programmed therein corresponding to one or more routines.Example control routines are described herein with regard to FIGS. 3-5.

Although not shown, engine 10 may also additionally include aturbocharger or supercharger system. A compressor may be coupledupstream of intake manifold 44 and downstream of throttle 62. Furtherembodiments include the compressor coupled upstream of throttle 62, thecompressor coupled either upstream or downstream of EGR pipe 28 and acharge after cooler (CAC) may be disposed within, or adjacent to, theintake passage 42, for cooling compressed air. Further still, a turbinemay be coupled downstream of exhaust 48 and upstream of emission controldevice 70.

Turning now to FIG. 2 an exploded view of example ammonia sensor 200 isshown. Ammonia sensor 200 includes a first insulating layer 210, anelectrolyte 230, and a second insulating layer 240 all disposed within ahousing, shown abstractly at dashed box 250. Ammonia sensor may furtherinclude temperature sensing elements (not shown), and a heater (also notshown) to increase performance of the sensing elements, such as layers210, 240, electrolyte 230 and further components included within ammoniasensor 200.

The first insulating layer 210, includes an NH₃ electrode 212, and NO₂electrode 216. First insulating layer 210 may be protected by porous toplayer 208. Electrode 212 is in physical and ionic communication with theelectrolyte 230 and can be disposed in fluid communication with a samplegas (e.g., engine exhaust gas). Electrode 212 catalyzes NH₃ gas toproduce electromotive force (EMF) and conducts electrical currentproduced by the EMF. In some examples, such an EMF may correspond to amagneto-resistance. Similarly, electrode 216 is in physical and ioniccommunication with the electrolyte 230 and can be disposed in fluidcommunication with a sample gas (e.g., engine exhaust gas). Also,electrode 216 catalyzes NOx gas to produce EMF and conducts electricalcurrent produced by such an EMF.

Electrode 212 is electrically coupled to contact pad 222 by electricallead 214. Likewise, electrode 216 is electrically coupled to contact pad226 by electrical lead 218. A reference electrode 242 is disposed on thesecond insulating layer 240 and further, electrode 242 is in physicaland ionic communication with the electrolyte 230. Reference electrode242 is coupled to contact pad 224 via trans-layer lead 232, secondarycontact 246 and electrical lead 244. Trans-layer lead 232 penetrateselectrolyte 230 and remains electrically isolated from electrolyte 230via insulation 234. Contact pads 222, 224 and 226 may be in electricalcontact with an example controller, and provide electrical signals basedon conditions at the electrodes 212, 242, 216 and electrolyte 230, suchan electric potential between any two electrodes, and/or acrosselectrolyte 230.

In the present example, exhaust is introduced to the electrode 212, thereference electrode 242 and electrode 216. The exhaust is diffusedthroughout the porous electrode materials. In the electrodes 212 and216, electro-catalytic materials induce electrochemical-catalyticreactions in the sample gas. These reactions includeelectrochemical-catalyzing ammonia and oxide ions to form diatomicnitrogen and water, electrochemical-catalyzing NO₂ to form NO, diatomicnitrogen and oxide ions (O⁻²), and electro-catalyzing NO and oxide ionsto form NO₂. Similarly, in the highly catalytic reference electrode 242,electrochemical-catalytic material induces electrochemical reactions inthe reference gas, primarily converting equilibrium oxygen gas (e.g.,diatomic oxygen) to oxide ions or vice versa. The reactions at theelectrodes 212, 242, 216 change the electrical potential at theinterface between each of the electrodes 212, 242, 216 and theelectrolyte 230, thereby producing an electromotive force (EMF).Therefore, the electrical potential difference between any two of thethree electrodes 212, 242, 216 can be measured to determine an EMF.

In further examples, an EMF generated corresponds to a potential betweentwo electrodes. In a first such example, the potential between theammonia electrode 212 and the reference electrode 242 may correspond toan ammonia concentration plus a NO₂ sensitivity, in a second suchexample, the potential between the NO₂ electrode 216 and the referenceelectrode 242 may correspond to a NO₂ concentration, and in a third suchexample, the ammonia electrode 212 and the NO₂ electrode 216 EMF can becompared to determine an ammonia and NO₂ concentration. In this way,measurable potentials (e.g., such as those measured at contact pads 222,224, and 226) may provide electrical signals that may be used tocalculate and/or derive concentrations of ammonia and NOx in theexhaust.

Because the primary reactants at electrode 212 are ammonia, water anddiatomic oxygen, the EMF generated at the electrode is due in part tothe partial pressures of each of these gases. Oxygen is measurable, forexample, using a lambda or exhaust gas sensor, such as at 126.Therefore, the remaining EMF signal can be attributed to ammonia andwater. Consequently, an ammonia concentration measurement may includethe partial pressure of water vapor in the exhaust. In this way, anammonia sensor can be used to infer humidity, such as ambient humidityof an ambient environment of a vehicle including the sensor in theengine exhaust. Similarly, NO₂ concentration may be measured using anEMF signal generated by reaction products at the NO₂ electrode 216.

FIG. 3 shows an example high level routine 300 for controlling ammoniaand humidity concentration measurements using an ammonia sensor. Theroutine may be executed in a vehicle control system, for example, andmay be repeated any number of times for real-time execution. During theroutine 300, a variable associated with time, T_(CURRENT), may be usedto track a number of iterations of the routine, and thus to track time.Additionally, the routine may include further or alternate methods oftracking time, such as time stamps, or reference to a clock.

In some examples, the routine 300 includes determining if thereindication of potential humidity change. Such an indication may bedetected, for example, at 312, 314 and 316 of routine 300, describedbelow. In the present example, routine 300 starts at 310 by determiningif there is an opportunity to measure humidity. Such an opportunity maybe indicated by a user-initiated change in an air-to-fuel ratio, or ananticipated change in air-to-fuel ratio. A user-initiated (e.g.,operator commanded) change in air-to-fuel ratio may be a tip-in or atip-out event such as input device 130, described above in FIG. 1. If anopportunity to measure humidity is detected the routine may continue to322.

Next, if it is determined that there is no opportunity to measurehumidity at 310, the routine 300 may continue to 312 to test ifT_(CURRENT) is greater than or equal to T_(PERIOD). T_(PERIOD) may be apredetermined amount of time between humidity measurements chosen toensure that the fastest envisioned change in humidity that may occur isdetected, like driving a vehicle that includes the example controlsystem, from a dry environment into a rain storm. If T_(CURRENT) isgreater than or equal to T_(PERIOD), then the routine continues to 322.

At 314, routine 300 includes assessing if there is an indication ofpossible humidity change from an accessory. An indication of a change inhumidity from accessories of the engine may include engaging ordisengaging windshield wipers (e.g., a windshield wiper condition), orturning on or off a component in a heating ventilation and airconditioning (HVAC) system (e.g., changing a vehicle climate controlparameter). The routine may check on a status of any such relevantaccessory, and may continue to 322 if a potential humidity change isindicated. If no accessory indicates a possible humidity change, thenthe routine may continue to 332.

If there is an opportunity to measure humidity at 310, or if T_(CURRENT)is greater than T_(PERIOD) at 312 or if the accessories indicate apotential humidity change at 314, then in response, the routine maycontinue to 322 which includes injecting an ammonia dose, the dose notbased on the ammonia sensor's reading. Because the ammonia sensor willbe used to measure humidity, and readings from the ammonia sensor maydetrimentally impact an ammonia sensor feedback control (as describedbelow, at the least in FIG. 4). However, because the engine may continueto produce NOx, ammonia and/or urea injection is continued. The doseinjected at 322 may be a preset amount or rate injected into the SCRsystem. Additionally, the dose injected at 322 may be a dose injected ina previous iteration of routine 300 (e.g., though ammonia injectiondosage is independent of current ammonia sensor feedback it may be basedon previous ammonia sensor feedback). Further, the dose injected at 322may be a feed-forward amount anticipated by engine conditions such asengine load, speed, mass air flow and the like. Further still, the doseinjected at 322 may be a feed-forward amount anticipated for a maximumair-to-fuel ratio used later in routine 300, for example at 324.

After 322, the routine may continue to 324, measuring exhaust humidityvia the ammonia sensor. One example of such a process is described inmore detail below as routine 500 in FIG. 5. After humidity has beenmeasured using the ammonia sensor, the T_(CURRENT) variable to zero. Inthe present example the double equal sign “==” is taken to meanassignment, as opposed to a logical test. After T_(CURRENT) has beenreset, the routine may end.

At 322, 324, and 326, the routine may be operating in a first mode, asshown by dashed box 320. In a first example, first mode 320 may includeat least one of recalibrating and measuring an exhaust humidity via anammonia sensor during two different air-to-fuel ratios. Further, in asecond example first mode 320 may include adjusting ammonia injectionindependent of the sensor while indicating ambient humidity based on thesensor. What is more, in the second example ambient humidity may bebased on a first ammonia sensor reading at a first lean air-fuel-ratioand a second ammonia sensor reading at a second lean air-fuel-ratio, theambient humidity further based on the first and second leanair-fuel-ratios. Engine airflow may be adjusted between a first engineairflow and a second engine airflow to generate the first and secondlean air-fuel-ratios, respectively, the engine airflow adjusted whilemaintaining fuel injection to maintain engine torque.

If there is no opportunity to measure humidity at 310, or if T_(CURRENT)is less than T_(PERIOD) at 312 or if the accessories do not indicate apotential humidity change 314, then the routine may continue to 332 tocontrol ammonia injection dosage. The box at 332 is shown with a dashedline box to indicate its optional nature, and may be excluded from someexamples of routine 300. One example of 332 is described below in FIG. 4at routine 400. After controlling ammonia injection dosage, the routineincludes incrementing variable T_(CURRENT) by one. Finally, the routinemay end after 334.

At 332, and 334 the routine may be operating in a second mode, as shownby dashed box 330. The second mode 330 includes controlling ammoniainjection dosage via feedback from the ammonia sensor, such as at 332and further described in routine 400, described below in FIG. 4.Further, the second mode may include adjusting ammonia injection inresponse to the ammonia sensor, and may additionally include correctingan amount of ammonia measured for ambient humidity. Further, second mode330 may include increasing or decreasing additional engine parameters,such as mass air flow, fuel injected or HVAC dehumidification inresponse to ambient humidity (as detected, for example, in first mode320 at 324). More generally, the inferred humidity may be used to modifyany engine parameter to optimize the engine for reducing emissions andimproving fuel economy.

In the present example of routine 300, box 320 indicates a first modeand box 330 indicates a second mode. However, in additional examples,box 320 may be a second mode and box 330 may be a first mode. Also, eachoperating mode may be include further operating modes. In one suchexample, second mode 330, includes a further first mode includingadjusting engine operation in response to an ammonia amount identifiedby the ammonia sensor and a further second mode including adjustingengine operation in response to an ambient humidity identified by theammonia sensor.

FIG. 4 shows an example routine 400 for controlling ammonia dosing to anSCR catalyst. Routine 400 is one example of a subroutine of high levelroutine 300 included at 332. However, in further examples, routine 400may run independent of routine 300. Further routine 400 is one exampleof instructions to during a mode, inject reductant based on a reading ofthe ammonia sensor.

Routine 400 includes, at 410 measuring an ammonia concentration. Asdescribed above, an example ammonia sensor may generate an EMF (whichmay be a concentration signal) that further may be correlated with apartial pressure of an engine's exhaust gas. If humidity and oxygenconcentration are known, for example by a lambda sensor or throughroutine 500 described below, the ammonia concentration in the gas may bededuced. In a further example, routine 400 includes adjusting theammonia reading from the sensor using humidity concentration and the airto fuel ratio in conjunction with a lookup table based on FIG. 6(described below) at 410 to deduce ammonia concentration.

At 412, routine 400 includes determining if ammonia concentration isbelow a first threshold. The first threshold may be a NOx emissionsthreshold, such that if ammonia concentration is below the firstthreshold, undesired NOx emissions may be released from the engine intothe atmosphere. If ammonia concentration is below the first thresholdthe routine may continue to 414 which includes increasing a dose amountof urea to be injected to the SCR catalyst by an incremental amount. Inalternate examples of the routine 400, the increase may be proportionalto some other measured engine condition, such as engine load, ratherthan being an incremental increase. After 414 the routine then continuesto 420 to inject the increased dose amount and then the routine may end.

If ammonia concentration is not below a first threshold at 412, theroutine may continue to 416, which includes determining if ammoniaconcentration is above a second threshold. The second threshold may bean ammonia slip threshold such that if ammonia concentration is abovethe second threshold, undesired ammonia may be released from the engineinto the atmosphere, and/or indicate excessive usage of reductant. Ifammonia concentration is above the second threshold the routine maycontinue to 418 which includes decreasing a dose amount of urea to beinjected to the SCR catalyst by an incremental amount. Similarly to 414discussed above, in additional examples of routine 400, decreasing theurea dose may be done proportional to other monitored engine parametersand conditions like engine load and exhaust temperature, instead ofbeing incremental. After 418 the routine then continues to 420 to injectthe decreased dose amount and then the routine may end.

Finally, if ammonia concentration is not above a second threshold and isnot below a first threshold, the routine may continue to 420, injectinga dose amount of urea. The dose amount may be the dose amount unchangedfrom a previous iteration of the routine 400 or may be a predefinedvalue. Further, the dose amount may be an anticipated amount based onengine measurements and conditions such as engine load, engine speed,fuel injection amount, air-to-fuel ratio, exhaust temperature, exhaustpressure and the like. In further examples of routine 400, urea may bereplaced by ammonia or other NOx reductants. Further, rates of injectionmay be used instead of amounts.

Turning now to FIG. 5 an example routine 500 for measuring humidity viaan ammonia sensor is shown. Routine 500 is one example of a subroutineincluded in high level routine 300 at 324. However, in further examples,routine 500 may run independent of routine 300.

Routine 500 is one example of a method for measuring humidity via anammonia sensor in an exhaust of an engine includes measuring a firstammonia concentration at a first air-to-fuel ratio, perturbing the firstair-to-fuel ratio to a second air-to-fuel ratio, measuring a secondammonia concentration at a second air-to-fuel ratio, and calculating ordetermining humidity based on a difference of the first ammoniaconcentration and the second ammonia concentration.

First, at 510, routine 500 includes determining if engine conditions arein a steady-state. Steady-state conditions may include constant exhaustflow, temperature, pressure, concentrations of ammonia, concentration ofNOx and other gas species. If the engine is not in a steady-state, theroutine may repeat the test until the engine is in a steady-state.Further examples of routine 500 may include processes, devices orsystems for bringing about steady-state engine conditions, beforereturning to 510. However, steady-state engine conditions are not arequirement for carrying out further actions in routine 500, as long asoxygen concentration is known. The boundary at 510 is dashed to indicatethe optional nature of the determination, and additional examples ofroutine 500 do not include 510.

Next, the routine continues to 512, which includes measuring a firstammonia concentration at a first air-to-fuel (A/F) ratio. As describedabove, an example ammonia sensor may generate an EMF (which may be aconcentration signal) by catalysis at an example electrode; such EMFsare then used to derive NH₃ concentration. Once a measurement hasoccurred, routine 500 may continue to perturb the first A/F ratio to asecond A/F ratio at 514. Perturbing the first A/F ratio may includegenerating an square-wave pulse width (PW) in the fuel injectors in theengine, and/or changing engine airflow. The square-wave PW may be asquare-wave pattern, and as a result of the square-wave PW, exhaustair-fuel-ratio may oscillate between a first exhaust air-fuel-ratio anda second exhaust air-fuel-ratio. Perturbing the first A/F ratio mayoccur in response to a user input or operator command, such as a tip-inor a tip-out event, as described above with respect to 310, FIG. 3.Further, the perturbation may bring about a faster change of percent O₂than percent change of NH₃.

Next, the routine includes measuring a second ammonia concentration at asecond A/F ratio at 516. Finally the routine includes calculatinghumidity based on a difference of a first ammonia concentration and asecond ammonia concentration at 518. The calculation may be done usingsignals generated by the given concentrations of gasses. In one examplea first ammonia concentration is measured when oxygen is 5 percent ofthe mass of the air (roughly 20:1 air-to-fuel ratio) and a secondammonia concentration is measured when oxygen is 14 percent of the massof the air (roughly 50:1 air-to-fuel ratio). As is shown in FIG. 6described below, an error results from ammonia concentrationmeasurements made at a given humidity.

In other words, the NH₃ concentration should stay relatively constantand because oxygen concentration is known during each of the ammoniaconcentration measurements (from either engine calculated percent O₂ oroxygen sensor measurements), then if there is a discrepancy between thefirst and second ammonia concentration measurements, the resulting errorcomes from humidity.

FIG. 6, discussed in more detail below, shows examples of such resultingammonia concentration measurements. If the humidity is dry (0 percentmass of water vapor in the exhaust at 610), the signal goes up as apercentage of diatomic oxygen goes up and the EMF signal goes down asthe percentage of diatomic oxygen goes down. If the humidity is wet (6%or 60 g of water per kg of air as shown at 670), the signal goes down asthe percentage of diatomic oxygen goes up and the signal goes up as thepercentage of diatomic oxygen goes down.

Returning to FIG. 5, in one example, a generalized error in the ammoniaconcentration signal is calculated (e.g., at 518) using an exampledelta:

$\frac{\begin{pmatrix}{{2^{nd}\mspace{14mu}{ammonia}\mspace{14mu}{concentration}\mspace{14mu}{signal}} -} \\{1^{st}\mspace{14mu}{ammonia}\mspace{14mu}{concentration}\mspace{14mu}{signal}}\end{pmatrix}}{\left. \sqrt{}\begin{pmatrix}{{\left( {\left( {2^{nd}\mspace{14mu}{ammonia}\mspace{14mu}{concentration}\mspace{14mu}{signal}} \right)\bigwedge 2} \right)/2} +} \\{\left( {\left( {1^{st}\mspace{14mu}{ammonia}\mspace{14mu}{concentration}\mspace{14mu}{signal}} \right)\bigwedge 2} \right)/2}\end{pmatrix} \right.}$Because of the dynamics of an example engine, under steady-stateconditions, a change of NH₃ concentration may be slow in comparison to achange of in percent O₂ (i.e. A/F ratio). Consequently, any change ofthe EMF from the NH₃ sensor may be substainally due to the percent O₂change and the intake humidity (g/kg H₂0). Further, the example deltaabove is independent of ammonia concentration and relies only on an EMFsignal generated by the ammonia sensor, and is independent of actualammonia concentration in the exhaust. Example delta is one way oftracking the partial pressure due to humidity in exhaust gas. Furtherthe calculated humidity may be an ambient humidity (as discussed above).

Once the routine has completed calculating the humidity based on thedifference of first and second ammonia concentrations at 518, theroutine may end. However, additional processes in routine 500 oradditional subroutines run in an engine control system may use thecalculated or inferred humidity value. Optionally, (as indicated by thedashed box) routine 500 may continue to 520 to adjusting an operatingparameter in response to the calculated ambient humidity. For example, amass air flow amount may be increased or decreased in response to theambient humidity Likewise, an amount of fuel injected into an exampleengine cylinder (e.g., a combustion chamber 30 described above withrespect to FIG. 1) may be increased or decreased in response to ambienthumidity. In a further example, an HVAC system may increase or decreasedehumidification in response to the detected humidity. In a stillfurther example, an EGR valve may increase or decrease the amount of EGRreturned to the engine in response to the detected humidity. In yet afurther example, readings of exhaust oxygen and/or NOx concentration maybe adjusted based on the detected humidity. Adjusting a given engineoperating parameter may be included as part of operating in an enginemode (e.g., second engine mode 330 described above with respect to FIG.3). Additionally, ammonia concentration may be adjusted usinginformation about detected humidity, such as at 410 in routine 400,described above with respect to FIG. 4.

In this way, ambient humidity may be measured via an ammonia sensor andthe detected value may be used to control and refine engine controlparameters, vehicle cabin climate, EGR systems and SCR systems. Furtherthe inferred measurement of humidity has the advantage of a lower costthan adding an extra sensor, and it is more reliable, since there arefewer parts.

Further, routine 500 is one example of instructions to during a mode,generate a first exhaust air-fuel-ratio, receive a second reading of anammonia sensor at the first exhaust air-fuel-ratio (e.g., at 512),generate a second exhaust air-fuel-ratio (e.g., at 514), receive a thirdreading of the ammonia sensor at the second exhaust air-fuel-ratio(e.g., at 516), determine ambient humidity based on a difference betweenthe second and third readings of the ammonia sensor (e.g., at 518), andinject the reductant based on the first reading of the ammonia sensor(e.g., at 322, as describe above in FIG. 3).

FIG. 6 is a graph illustrating example relationships between error inhumidity measured by one example ammonia sensor and air-to-fuel ratiosfor various amounts of humidity. Each curve represents a constanthumidity, measured in grams of water vapor to kilograms of air. Curve610 is a dry curve (0 g/kg), curve 620 is at 3 g/kg, curve 630 is at 7g/kg, curve 640 is at 10 g/kg, curve 650 is at 20 g/kg, curve 660 is 40g/kg, and curve 670 is at 60 g/kg. Based on such trends, an example map,look-up table or similar calculation may be used to determine humidityfrom signals associated with ammonia sensor concentrations, for exampleas done in 518 in FIG. 5.

Continuing with FIG. 6, in one example of curve 610, dry air enters anexample intake (e.g., intake passage 42), and the air to fuel ratio(A/F) is increased form 30:1 to 70:1 in a step-wise fashion, an EMFsignal rises from −5 mV up to +25 mV. Such a change indicates that theambient air is dry. By comparison, a higher ambient humidity (forexample curve 670 with 60 g H₂0/kg air), a measurement remains constantat −25 mV before and after a perturbation to the A/F ratio such as anexample percent O₂ square wave oscillation (e.g., 30:1 to 70:1 change ofA/F ratio). In this way, a constant EMF measurement from an example NH₃sensor may indicate humidity.

Further, an intake temperature reading (for example, as monitored by_121), combined with psychometric saturation values (which, in someexamples, are stored, as a table in a memory of controller 12) may beused to check and/or correct the values of example maps, look-up tablesor similar calculations used to determine humidity from signalsassociated with ammonia sensor concentrations (as described above). Inone example an intake air temperature is very cold, such as −50 degreesFahrenheit, the psychometric saturation value of humidity is 0.02 g/kg.Hence, a change in EMF should be close to that predicted by line 610. Inan additional example, an intake air temperature reading of 90 degreesFahrenheit may have a psychometric saturation value at 31 g/kg, orbetween lines 650 and 660 of FIG. 6. So, the expected change of EMF fromthe NH₃ sensor should be small for a given A/F perturbation, such aswith a percent O₂ square wave. Additionally intake air temperature maybe combined with an indication that a vehicle accessory is active (suchas windshield wipers) to may indicate humid ambient conditions.Consequently, checking during various temperature conditions may confirmor disconfirm the accuracy of curves used to calculate humiditymeasurements, such as 610, 620, 630, etc. of FIG. 6.

Further, still calibration curves, such as those shown in FIG. 6 (e.g,at 610, 620, etc.) may be dependent on the NH₃ sensor design and how itis produced. Therefore, the A/F and humidity curve may be specified andnot changed over time. Alternatively, if it is determined that thedesign or production of the NH3 sensor results in variable of thecurves, on a part-to-part basis, then a part identification of anexample sensor or a connector of the sensor may determine a set of A/Fand humidity calibration curves. For example, a trimmed resistor in theconnector or a simple memory chip in the connector could be detected byan example control system (e.g, control system 14). In one example, thecontroller would reference a look up table for that particular NH₃sensor is connected and have the correct A/F and humidity chart to findambient humidity.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and nonobvious combinationsand subcombinations of the various systems and configurations, and otherfeatures, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application.

Such claims, whether broader, narrower, equal, or different in scope tothe original claims, also are regarded as included within the subjectmatter of the present disclosure.

The invention claimed is:
 1. A method for an engine having an exhaustwith an ammonia sensor, comprising: adjusting an operating parameter inresponse to ambient humidity, the ambient humidity based on a firstammonia sensor reading at a first exhaust air-fuel-ratio and a secondammonia sensor reading at a second exhaust air-fuel-ratio.
 2. The methodof claim 1 wherein the operating parameter is an engine parameter, andwhere the first ammonia sensor reading is a first ammonia concentration,and the second ammonia sensor reading is a second ammonia concentration,where an exhaust air-fuel-ratio oscillates between the first exhaustair-fuel-ratio and the second exhaust air-fuel-ratio.
 3. The method ofclaim 2 wherein the ambient humidity is based on a difference betweenthe first ammonia concentration and the second ammonia concentration,and further based on a partial pressure of water vapor in the exhaust.4. The method of claim 1 wherein an engine exhaust flow rate issubstantially at steady-state when identifying the first ammonia sensorreading and the second ammonia sensor reading, the first air-fuel-ratiodifferent from the second air-fuel-ratio, the first air-fuel-ratio beinglean and the second air-fuel-ratio being lean.
 5. The method of claim 1further comprising perturbing the exhaust air-fuel-ratio between thefirst exhaust air-fuel-ratio and the second exhaust air-fuel-ratioduring steady exhaust flow conditions by adjusting fuel injection in asquare-wave pattern, and while an operator command is steady.
 6. Themethod of claim 1 further comprising perturbing the exhaustair-fuel-ratio between the first exhaust air-fuel-ratio and the secondexhaust air-fuel-ratio in response to an operator commanded tip-in ortip-out.
 7. The method of claim 1, where the ammonia sensor comprises: afirst insulating layer comprising an ammonia electrode and a NOxelectrode, the ammonia electrode coupled to a first contact pad and theNOx electrode coupled to a second contact pad; a second insulating layercomprising a reference electrode coupled to a further contact pad; andan electrolyte, each electrode in physical and ionic communication withthe electrolyte, and the first layer, second layer and electrolyte alldisposed within a housing.
 8. The method of claim 1, where the ammoniasensor is located downstream of a selective catalyst reduction catalyst,the catalyst receiving urea via urea injection.
 9. A method using anammonia sensor in an engine exhaust SCR system, comprising: in a firstmode, identifying ambient humidity by measuring an ammonia concentrationvia the ammonia sensor during two different air-fuel-ratios, themeasuring in response to an indication of a potential humidity change.10. The method of claim 9 further comprising: in a second mode,controlling an ammonia injection dosage via feedback from the ammoniasensor, where during the first mode, ammonia injection dosage isindependent of current ammonia sensor feedback and based on previousammonia sensor feedback.
 11. The method of claim 9, where the indicationof potential humidity change is based on a vehicle climate controlparameter.
 12. The method of claim 9, where the indication of potentialhumidity change is based on a windshield wiper condition.
 13. A methodfor an engine having an ammonia sensor in an exhaust downstream of anSCR catalyst, comprising: during a first mode, adjusting ammoniainjection in response to the sensor; and during a second mode, adjustingammonia injection independent of the sensor while indicating ambienthumidity based on the sensor.
 14. The method of claim 13 wherein theengine is a diesel engine and, during the second mode, ambient humidityis based on a first ammonia sensor reading at a first leanair-fuel-ratio and a second ammonia sensor reading at a second leanair-fuel-ratio, the ambient humidity further based on the first andsecond lean air-fuel-ratios.
 15. The method of claim 14 wherein engineairflow is adjusted between a first engine airflow and a second engineairflow to generate the first and second lean air-fuel-ratios,respectively, the engine airflow adjusted while maintaining fuelinjection to maintain engine torque.
 16. A method using an ammoniasensor in an engine exhaust SCR system, comprising: during a first mode,adjusting engine operation in response to an ammonia amount identifiedby the ammonia sensor; and during a second mode, adjusting engineoperation in response to an ambient humidity identified by the ammoniasensor.